Spinel and process for making same

ABSTRACT

This invention pertains to product and process. The product is a transparent product of a density in excess 99.5% comprising spinel and having uniform mechanical properties. The process pertains to fabrication of a transparent spinel product comprising the steps of dissolving a sintering aid in water to form a neutral sintering aid solution, adding a suitable additive to the sintering aid solution, applying the sintering aid solution to spinel particles to form a spinel dispersion, sub-dividing or atomizing the spinel dispersion to form droplets comprising one or more spinel particles coated with the final spinel solution, drying the droplets to form dried coated particles comprising one or more spinel particles coated with a dried layer of the sintering aid, and densifying the dried coated particles to form a transparent spinel product having uniform optical and mechanical properties in absence of grains of exaggerated size.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention pertains to the field of sintered ceramics,particularly magnesium aluminum spinel, and to a process for preparingsintered ceramic articles from ceramic powders.

[0003] 2. Description of Related Art

[0004] Sintering is defined as the act of consolidating powder into adense shape. The powder being sintered must additionally not melt to agreat extent, some melting of secondary phases in the powder, or surfacemelting is allowed under this definition. If the material completelymelts, the process is referred to as fusion casting. Sintering, bothpresureless and with pressure, or hot pressing, requires solid, liquidor gas material transport to consolidate an aggregate of loose powderparticles into a dense shape. In the case of porcelains and clayproducts, secondary phases do melt and “glue” the primary solidparticles together with a glassy phase. These types of systems were thefirst to be used due to their ease of sintering. However, advancedceramics do not have these intrinsic sintering aids and they musttherefore, be added. For small samples, the powdered sintering aids aremixed with the powder to be sintered with a mortar and pestle. In largersamples, mixing is accomplished by ball milling, attritor milling, highshear wet milling, and variations or combinations of these methods.

[0005] Spinel is defined as a crystalline structure of the type AB₂O₄where A is a 2+ cation occupying tetrahedral lattice site in an oxygencubic close packed structure and B is a 3+ cation occupying octahedrallettice site. In a preferred embodiment, spinel is MgAl₂O₄ consisting ofan oxide of magnesium and aluminum. Spinel powder can be prepared by wetchemistry, solid state diffusion of oxides or calcination. Spinel powderparticles consist of crystallites which are less than 500 nm in sizethat can also be agglomerated into larger sizes varying from 500 nm to100 μm, more typically 1-50 μm.

[0006] Spinel is important because it is strong and transparent fromvisible to 5.5 μm wavelength. Its mechanical properties are severaltimes greater than that of glass and make it a leading candidate for useas a transparent armor and window material. Commercially, it can be usedas a stronger and thinner window for many applications including lap topcomputers, cell phones, automotive glassing and headlamps, aerospacewindshields, and industrial blast shields.

[0007] Dense, transparent spinel articles are not currently availablefrom a commercial source although there are companies currently tryingto develop a viable manufacturing process. Since there is no viablemanufacturing process, the cost of spinel products is so high that eventhe military avoids its use.

[0008] Difficult to sinter materials, such as spinel, are typicallymixed with a sintering aid or a secondary material that aids indensification. The sintering aids work in a variety of fashions. Thesintering aids may liquefy at or somewhat below the primary material'sdensification temperature thereby promoting liquid phase sintering.Certain sintering aid materials exhibit higher solid-state diffusioncoefficients than the primary material's self-diffusion coefficient. Thesecondary material may conversely have a lower solid-state diffusioncoefficient that prevents exaggerated grain growth and promotes grainboundary refinement and pinning. The sintering aid may also simply cleanor etch the primary material's surfaces thereby enhancing solid-statediffusion. These are broad examples of the mechanisms by which sinteringaids enhance densification. In actual practice, sintering aids may notfit into just one of the categories outlined and the same aid may havedifferent functions in different material systems, or have no effect inother systems.

[0009] Sintering aids tend to be solid inorganic particles at roomtemperature. Sintering aid particles henceforth are defined ascomprising crystallites (≦500 nm), crystals (>500 nm), and agglomeratesof crystallites and/or crystals. Since the materials to be densified aregenerally also solid inorganic particles, the two materials must bemixed homogeneously for the sintering aid to be effective. This isaccomplished by some form of mechanical mixing. However, due to thenature of particle-particle interactions, the mixture is far fromhomogeneous. Inhomogeneity in the mixture results in areas that have toomuch sintering aid and other areas that have little or no sintering aid.This is a major problem in the fabrication of transparent ceramics,electronic ceramics, and in high tech refractory ceramics.

[0010] The Sellers et al U.S. Pat. No. 3,768,990 discloses an opticalelement having transparency in the visible and infrared wave lengthsthat is made by heating at an elevated temperature a composition havingsub-micron particle size of magnesium oxide and aluminum oxide havinguniformly mixed therethrough 0.2-4% by weight of powdered LiF. It isbelieved that optical and mechanical properties of the Seller's opticalelement are negatively impacted by the inhomogeneous presence ofsubstantial amount of LiF. This leads to microstructural regions thatare highly porous and other microstructural regions that exhibitexaggerated grain growth, all of which lead to inferior optical andmechanical properties. This has prevented the use of spinel in practicalapplications since the Seller's patent issued in 1973. Furthermore, itis believed that the atomic concentrations of lithium and fluorine willbe greater than about 1000 ppm and 100 ppm, respectively due to the factthat LiF is well known to react with alumina, which Seller's uses as astarting powder.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION

[0011] It is an object of this invention to make spinel products orproducts that contain spinel that are moderately priced.

[0012] It is another object of this invention to increase transmissionof spinel products from about 5% at 0.525 μm, which is relativelyopaque, to at least 50% at 0.525 μm, which is transparent.

[0013] It is another object of this invention to eliminate or reduce theHot Isostatic Pressing (HIP) procedure in the fabrication of the spinelproducts.

[0014] Another object of this invention is the more uniform distributionof the sintering aid on the spinel particles.

[0015] Another object of this invention is to increase densification ofsintered spinel products beyond 99.8%.

[0016] Another object of this invention is the more complete and moreuniform covering on surfaces of the spinel particles by a sintering aidwhich is achieved by dissolving the sintering aid in a solvent to createa sintering aid solution and then adding the spinel particles in thesintering aid solution to form a dispersion, also referred to as aceramic dispersion.

[0017] These and other objects of this invention can be achieved by atransparent spinel product that is fabricated by sintering anddensifying spinel powder in absence of the HIP procedure by dissolving asintering aid in a solvent to form a sintering aid solution, addingspinel particles in the sintering aid solution to form a dispersion,maintaining the dispersion in a state which discourages precipitation ofthe solid sintering aid, spray-drying the dispersion to form spinelparticles coated with a sintering aid, and densifying the coated spinelparticles to form a transparent product.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic illustration of the process of coatingspinel particles with LiF sintering aid and formation of the finaldensified MgAl₂O₄ product;

[0019]FIG. 2 is a schematic illustration of a preferred spray-dryingsystem;

[0020]FIG. 3 is a graph of percent (%) Transmission versus Wavelength ofdensified spinel products wherein the curve marked #1, representingprior art, is mechanically mixed spinel particles with 0.5% by weightLiF sintering aid as opposed to spray-dried spinel particles with 0.5%by weight (curve #2) and 2.0% (curve #3) LiF sintering aid, whichrepresent the invention herein. Curve #4 represents theoreticaltransmission.

[0021]FIG. 4 is a representation of three discs made from coated spinelparticles and correspond to curves #1, #2 and #3 in FIG. 3 showingpictorially opaque disc #1, which represents prior art, and transparentdiscs #2 and #3, which represent this invention.

[0022]FIG. 5 is a table showing relative parameters of spinel (MgAl₂O₄)and glass;

DETAILED DESCRIPTION OF THE INVENTION

[0023] This invention pertains to a sintered and transparent spinelproduct and to a process for making it which is characterized byspraying a dispersion consisting of spinel particles in a sintering aidsolution to form spinel particles coated with the sintering aid. Thesintering aid coating on the spinel particles need not be continuous,although it should be sufficient to prevent a large number of siteswhere the particles contact each other without an intervening layer of asintering aid.

[0024] The sintered and densified spinel product of this invention isnovel and unobvious when compared to prior art. Preparation of a spinelproduct by prior art procedure yields a product that is not transparentbut opaque afrer hot pressing and requires extended HIPing to render theproduct transparent, however, with consequent loss of optical andmechanical properties. To explain more fully, the prior art product canbe made transparent by extending the HIPing procedure beyond thecustomary period of on the order of a day, however, prolonging the hotprocessing and/or the HIPing procedure introduces exaggerated graingrowth which renders the resulting product non-uniform in terms ofoptical and mechanical properties such as elastic modulus, flexurestrength and fracture toughness, properties which are paramount forspinel character. The novel and unobvious product, made in absence ofthe HIP procedure, as described herein, is transparent over thewavelength range of about 0.3-5.5 μm, has uniform optical and mechanicalproperties and its grains are smaller than about 1 mm, typically smallerthan about 500 μm. The spinel product of this invention does not havethe exaggerated grain growth or is essentially devoid of grains ofexaggerated size. There are no grains larger than about 1 mm.Exaggerated grain growth typically leads to grains that are greater than3 times larger to several orders of magnitude larger than an averagesized grain. Transparency of the spinel product of this invention, madein the manner disclosed herein and in absence of the HIP procedure, isabove about 50% and up to about 90%.

[0025] In the preparation of the sintered spinel product of thisinvention, the sintering aid, such as LiF or any other suitablesintering aid such as NaCl, NaF, LiCl, etc., is dissolved in a suitablesolvent, typically water, to form a sintering aid solution. Thesintering aid can be in any condition, however, it is typicallyparticulate with particle sizes in the range of 500 nm-10 μm. This, ofcourse, is not important because the sintering aid is eventuallydissolved to form a sintering aid solution. To enhance spraying of thesintering aid solution, ethanol or isopropanol or another suitablediluent is admixed with the aqueous sintering aid solution to form amodified sintering solution, which henceforth is referred as thesintering aid solution. Aqueous solution of LiF by itself is not readilysprayable using an ultrasonic atomizer in absence of a low surfacetension/viscosity liquid component, which is typically a suitableadditive, such as an alcohol. The overriding consideration in addinganother component to water is to enhance solubility of the sintering aidand the sprayability of the modified sintering aid solution. If thesintering aid is other than LiF, other component(s) known to a personskilled in the art may be used. A typical sintering aid solution isprepared by admixing 0.2 g LiF sintering aid, 220 ml water, and 780 mlethanol or another like component. Typically, the ratio of thecomponents is on this order of magnitude. The sintering aid solutionshould not contain too much sintering aid, such as about in excess ofabout 10% by weight.

[0026] Coating of the spinel particles can be effected in any desiredmanner in order to deposit a uniform layer of the sintering aid on theparticles. The sintering aid coating can be applied onto the spinelparticles in any suitable manner such as in a fluidized bed, by a wetchemistry technique, by CVD, plasma enhanced CVD, laser assisteddeposition, by sputtering, by an evaporation technique, and the like.The coating need not be continuous, but should be sufficient to preventa large number of sites where the particles contact each other withoutan intervening layer of a sintering aid material. Spraying of the finalsintering aid solution can also be used to deposit at least a monolayerof the sintering aid on the spinel particles. Another way ofaccomplishing this objective is simply to immerse the spinel particlesin the final sintering aid solution until the spinel particles acquire acoating of the sintering aid of sufficient thickness and sufficientuniformity.

[0027] The sintering aid solution is maintained in a state where it ison the spinel particles and the sintering aid is in solution and notprecipitated on the spinel particles. This may require adjustment oftemperature, pH, and/or another parameter(s) to discourage precipitationof the sintering aid on the particle surface. If the sintering aid isLiF, precipitation thereof can be discouraged or prevented bymaintaining a neutral pH of about 7 of the sintering aid solution.

[0028] The spinel particles are typically in the range of 500 nm to 100μm and amount of the sintering aid on the spinel particles is typically0.05-10% on weight basis, more typically 0.1-2% by weight of the spinelparticles. FIG. 1 is illustrative of the coating process and showsspinel particles 12 coated with sintering aid 14, sintered to produceproduct 16. Spinel particles must be insoluble in mixtures of water andthe additives used.

[0029] The dispersion resulting when spinel particles are mixed with thesintering aid solution is delivered to an atomizer where the dispersionis sprayed, causing sub-division into droplets which are transportedinto a drying zone where vaporizable matter is removed from the dropletsand the solid coating is formed thereon. The droplet size can be up to1000 μm but is typically up to 500 μm, more typically 5 nm to 250 μm,and especially 50 nm to 50 μm. Any suitable atomizer can be used,including mechanical, piezoelectric (ultrasonic) and electrostatic, aslong as droplets containing the desired number of spinel particles areformed and the preponderance of resulting coated droplets are completelyor hermetically sealed or coated.

[0030] Whatever atomizer is used, chemistry of the dispersion should besuch as to prevent premature precipitation of the coating on the spinelparticles, and the droplets issuing from the atomizer should contain atleast one of the spinel particles per droplet. Typically, an ultrasonicatomizer is used at a variable frequency since size of a droplet can becontrolled by varying atomizer frequency. For instance, at atomizerfrequency of 20 kHz, droplets of about 90 μm can be formed; at frequencyof 40 kHz, droplets of about 45 μm can be formed; and at 80 kHz,droplets of about 20 μm can be formed. As is apparent, the inverserelationship between atomizer frequency and the droplet size can be usedto control the droplet size.

[0031] The higher the temperature in the drying zone, the shorterresidence time is required of the droplets in the zone to have thevaporizable matter removed from and the coating formed thereon. Thetemperature in the zone should be high enough to drive-off volatilematter from the particles but not so high as to impart thermal damage tothe particles or the coating. Furthermore, temperature in the zoneshould be high enough to drive-off volatiles from the droplets in areasonable or desired time, which can be adjusted by changingtemperature in the zone, with higher temperature in the zone reducingresidence time of the droplets to form dry, coated particles. Typically,depending on many factors, temperature in the drying zone should be inexcess of about 100° C. and below 600° C., more typically 200° C.-500°C.; and speed of the droplets through the drying zones in the dropletdirection is typically 0.1-1000 cm/sec, more typically 50-500 cm/sec.When moving in the zone, the droplets are entrained in hot air or in aninert gas or a reactive gas. Residence time in the zone is instantaneousto a fraction of a minute, typically 0.1-10 seconds.

[0032]FIG. 2 illustrates a preferred spray drying system 210 composed ofa pair of 5-foot long silica tubes of ⅛ of an inch in wall thicknessjoined end to end to form a continuous vertically disposed conduit about10 feet long. The spinel dispersion is taken to an atomizer andintroduced into the drying system through the top. Three heat or dryingzones 212, 214, 216, are arranged around the conduit. The first heatingzone 212 is disposed around the upper portion of the conduit but about ¾of a foot below the top of the conduit where it maintains a temperatureof 150° C. within the conduit; the second heating zone 214 is disposedaround the lower portion of the conduit and spaced 5 feet below thebottom of the first heating zone where it maintains a temperature of350° C. within the conduit; and the third heating zone 216 is disposedaround the bottom portion of the conduit and spaced half a foot belowthe bottom portion of the second heating zone where it maintains atemperature of 430° C. Below the third heating zone 216 is a 2-foot longsection 218 that is a continuation of the conduit and below section 218is a half-foot long conical section 220 which terminates in an opening 2inches in diameter. If temperature in the sections is not sufficientlyhigh, LiF sintering aid will remain wet and the droplets will sticktogether, however, if the droplets are dried too fast, the deposited LiFwill spall off the spinel droplets. If the given temperature profiledisclosed herein is adhered to, satisfactory drying in a reasonable timeis attained.

[0033] Below conical section 220 is cyclone separator 222 wherein thecoated particles (dried droplets) are separated from the gas stream andleft in collection bin 224. Suction hose 226 transports the gaseouscomponents to exhaust. Coated particles from the collection bin aretaken to densification.

[0034] The spray-dried coated particles, which can contain at least onespinel particle, are hot-pressed in an inert atmosphere at about 1550°C. and 5000 psi for about 2 hours to obtain a transparent monolithicsintered spinel product of less than 0.2% porosity. When in the hotpress, minimal pressure of about 50 psi is initially applied until thecoated spinel powder starts to densify at about 1100° C. and thereafter,pressure is raised to about 5000 psi when a temperature of about 1450°C. is reached. In a preferred embodiment, the heating schedule in thisdensifying procedure, which is conducted under initial vacuum of about10⁻⁴ Torr, includes about a 20° C./minute ramp from ambient temperatureto 950° C., about a half hour hold to allow the sintering aid LiF tomelt and clean/etch the spinel particle surfaces, another 20° C./minuteramp to 1200° C., another half hour hold to allow vaporized sinteringaid to leave the hot press, still another 20° C./minute ramp to about1550° C., a 2-hour hold to fully densify the spinel powder into atransparent shape, and turning off the heating elements and thehydraulic pump to allow pressure to bleed-off and temperature of theproduct in the hot press to cool to about room temperature under avacuum of less than 10⁻³ Torr. Melting point of LiF is 850° C. and itsboiling is about 2000° C. but starts vaporizing above its melting point.The heating and pressure schedule can be modified to accommodate othersintering aids.

[0035] In densifying the coated particles, it is important to keep inmind the necessity of removing the sintering aid, such as LiF, at thelowest possible temperature below about 1000° C. to a level below about500 ppm lithium from the sintering aid LiF, more typically below about100 ppm lithium, and below about 50 ppm fluorine from the sintering aidLiF, more typically below about 10 ppm fluorine, hereafter, referred toas essentially devoid of the sintering aid components, in this caselithium and fluorine. It is necessary to abide by these limits since athigher temperatures, reaction between the sintering aid and spinel ismade more likely with the imminent production of impurities, such aslithium aluminate, lithium aluminum oxyfluoride and lithium aluminumfluoride which may negatively affect properties, such as the optical andmechanical properties.

[0036] Although the HIP procedure is typically omitted in this novel andunobvious process, and thus the cost of resulting product is reduced by⅓ to ½, it may be desirable to subject the product of this invention tothe HIP procedure of short duration to positively alter transparency,porosity or some other parameter(s) of the product. The HIP procedurefor a sintering aid, like LiF, is carried out in an inert atmosphere attemperature above 1500° C., such as about 1750° C., and at a pressureabove 20,000 psi, such as about 30,000 psi, to obtain a transparentproduct of porosity on the order of less than 0.2%. Duration of the HIPprocedure in the past has been 1 day, however, with the process of thisinvention, HIP duration may be as short as about 1 hour and is typically1-4 hours, when used. Generally, density of spinel products should begreater than 99.5% to have transparency since such products with densityof less than about 99.5% are milky white and opaque.

[0037] The novel process disclosed herein is unobvious since it yields atransparent product after hot pressing in absence of the HIP procedure;reduces hot pressing conditions, provides for better reproducibility;increases yield; provides for smaller average grain size; reducesporosity; and minimizes exaggerated grain growth. Generally, the smallerand the more uniform grain size, the better the optical properties, upto a point. Once the particles become too small, such smaller than about0.05 μm, strength, and possibly other parameters are negativelyimpacted.

[0038]FIG. 3 is a graph which shows variation of transmission atdifferent wavelengths of light with the manner of preparing sintered anddensified spinel experimental disks which were 1″ in diameter and 1 mmin thickness. The disks were polished, first with SiC and then withdiamond before use. The curves in FIG. 3 are identified as #1, #2, #3and #4. Curve #1 was made by testing disks prepared by mechanical mixing0.5% by weight of the sintering aid LiF with spinel particles, and isnot representative of the invention herein: curve #2 was made by testingdisks prepared by spraying 0.5% by weight of the sintering aid LiFpursuant to the invention herein; curve #3 was made by testing disksprepared by spraying 2% by weight of the sintering aid LiF pursuant tothe invention herein; and curve #4 represents theoretical or a solidmonolith of spinel MgAl₂O₄ where porosity was essentially zero. Data forthe theoretical curve #4 was obtained from literature. Spinel powderparticles that were used to prepare disks #1, #2 and #3 correspond tocurves #1, #2 and #3 of FIG. 3, were in the range of 500 nm-10 μm. Inpreparing disk #1, the spinel particles and the particulate LiFsintering aid were mixed in a mortar and pestle for about 5 minuteswhereas for disks #2 and #3, a sintering aid (LiF) solution wasprepared, as described above, and used to spray-dry a LiF coating on thespinel particles. For disks #1, #2 and #3, densification in a hot presswas carried out at initial vacuum of 10⁻⁴ Torr pursuant to the followingschedule: 20° C./minute ramp from ambient to 950° C.; 30-minute holdperiod; another 20° C./minute ramp from 950° C. to 1200° C.; another30-minute hold period; another 20° C./minute ramp from 1200° C. to 1550°C.; 2-hour hold period; and an extended cool down and pressure bleed-offperiod. Transparency or opacity of the disks corresponding to the curves#1, #2 and #3 is illustrated in FIG. 4 where disk #1 is shown as beingopaque, disk #2 is shown as being partially transparent, and disk #3 isshown as being transparent. From FIG. 3, it is apparent that in thevisible region of 0.4-0.7 μm, only disk #3 reaches transparency in thearea of about 55%. It should be noted that, based on the data for curve#4 in FIG. 3, transmission drops from about 80% to about nil at awavelength of about 0.3 μm.

[0039] The coating thickness on the particles can be varied, inter alia,by adjusting dilutions of the coating solution and/or by adjustingfrequency of the atomizer; if an ultrasonic atomizer is used. Forpurposes herein, it has been found that coating thickness in the rangeof 1-1000 nm, more typically 2-200 nm is suitable. Uniformity of coatingthickness was confirmed by scanning electron microscopy.

[0040] FIGS. 5 shows superior properties of the magnesium aluminumspinel product compared to glass.

[0041] Having described the invention, the following examples are givenas particular embodiments thereof and to demonstrate the practice andadvantages thereof. It is undersood that the examples are given by wayof illustration and are not intended to limit the specification or theclaims in any manner.

EXAMPLE 1

[0042] This example details the use of lithium fluoride (LiF) sinteringaid as a coating on magnesium aluminate (MgAl₂O₄) spinel particles. Theuse of LiF coating allows the hot pressing of the coated spinelparticles in an inert atmosphere into a sintered transparent shape thathas 70% transmission, as is detailed in Ex. 2.

[0043] The sintering aid in this example was 0.2 grams of LiF powderwith a particle size in the range of 500 nm-10 μm. The LiF was initiallydissolved in 220 ml of deionized water by mixing for about a quarter ofone hour followed by addition of 780 ml of ethanol with mixing for aboutanother quarter of one hour.

[0044] Ten grams of the spinel powder was mixed with the LiF sinteringaid solution to form a dispersion that was delivered to an ultrasonicatomizer at a rate of 30 ml/minute using a metering pump and sprayed.The LiF sintering aid solution was at a neutral pH of about 7 whichprevented LiF dissolved in the solution from precipitating on the spinelparticles in the dispersion. The atomizer was operated at a frequency of40 kHz and produced a fine stream of 45-micron droplets, containing 1-3spinel particles, that were passed through a drying system illustratedin FIG. 2 at a rate of about 9 cm/second before being collected in acyclone separator to form dried coated particles.

[0045] The droplets from the atomizer were introduced into the dryersystem where the first drying zone was 1 foot from the top andtemperature therein was 150° C. The second drying zone was 4 feet frombelow the first and was also 1 foot in length, as were all the others,but its temperature was 350° C. The third drying zone was ½ foot belowthe second and its temperature was 430° C. Below the third drying zone,there was a 2-foot straight section followed by a three quarter of afoot conical section. The dried coated particles issuing from theconical section through a 2-inch opening were directed to a cycloneseparator where the dried coated particles were separated and kept in abin and the gaseous components were removed through the suction hose.The coated particles from the bin were later removed to be densified.The coated particles were characterized by X-ray diffraction, scanningelectron microscopy, and x-ray fluorescence. The coated particles werecharacterized as containing spinel (MgAl₂O₄) particles with a LiFcoating.

EXAMPLE 2

[0046] This example provides details as to densification of the driedcoated spinel particles prepared in the manner described in Ex. 1,above. The product had transmission of 70% and was prepared in a mannerthat did not include the expensive HIP procedure.

[0047] The coated particles in powder form and prepared as described inEx. 1, above, were placed in grafoil-lined graphite hot press die andthe die was placed in an inert argon atmosphere (or a vacuum of 10⁻⁴Torr). Minimal pressure was applied until the powder started to densifyat about 1100° C. and pressure was stepped up to about 5000 psi when thetemperature of about 1450° C. was attained. The heating scheduleincluded a 20° C./minute ramp from ambient to 950° C., a 30-minute holdto allow LiF to melt and clean/etch the spinel particle surfaces, a 20°C./minute ramp from 950° C. to 1200° C., a 30-minute hold to allowvaporized LiF, and probably other components, to escape the hot pressdie, a 20° C./minute ramp to 1550° C., and a 2-hour hold to fullydensify the spinel powder into a transparent shape. The heating elementsand the hydraulic pump were then turned off to allow natural cooling ofthe hot press and allow the pressure to bleed-off, which took about 3hours.

[0048] While presently preferred embodiments have been shown of thenovel and unobvious sintered spinel products and their preparation,persons skilled in this art will readily appreciate that variousadditional changes and modifications can be made without departing fromthe spirit of the invention as defined and differentiated by thefollowing claims.

What is claimed
 1. A product that is essentially devoid of a sinteringaid components comprising spinel that has porosity of less than 0.2%, istransparent to light having wavelengths in the range of 0.4-5.5 microns,has uniform properties, is devoid of grains larger than about 1 mm andis devoid of grains of exaggerated size.
 2. The product of claim 1wherein its spinel grains are less than about 300% of the average-sizedgrain.
 3. The product of claim 1 having transparency in excess of 50%and its spinel grains are within about 300% of the size of an averagegrain.
 2. The product of claim 1 wherein its spinel grains are less thanabout 300% of the average-sized grain.
 4. The product of claim 3 havingtransparency of at least 60% at a wavelength of 4 microns and the spinelis a hard crystalline solid selected from the group consisting of oxidesof magnesium and aluminum.
 5. The product of claim 2 having transparencyof at least 60% at a wavelength of 4.0 μm and the spinel is a hardcrystalline solid MgAl₂O₄.
 6. A process for preparing a transparentceramic product comprising the steps of: (a) dissolving a sintering aidin a suitable solvent to form a sintering aid solution, (b) applying thesintering aid solution to ceramic particles to form a ceramicdispersion, (c) sub-dividing the ceramic dispersion to form dropletscomprising at least one ceramic particle coated with the sintering aidsolution, (d) drying the droplets to form dried coated particlescomprising at least one ceramic particle coated with a dried layer ofthe sintering aid, and (e) densifying the dried coated particles to forma transparent ceramic product having uniform optical and mechanicalproperties and being devoid of grains larger than about 1 mm and grainsof exaggerated size.
 7. The process of claim 6 wherein said densifyingstep is carried by applying minimal pressure of about 50 psi to thecoated particles while raising temperature to above the meltingtemperature of the sintering aid and pressing the dried coated particlesat above about 5000 psi while increasing temperature to above about1500° C.
 8. The process of claim 6 wherein said ceramic particles arespinel MgAl₂O₄, wherein said densifying step is accomplished in a hotpress by ramping temperature from ambient to above 1500° C.
 9. Theprocess of claim 6 wherein said ceramic particles are spinel MgAl₂O₄,and wherein said densifying step is accomplished by ramping steps to anelevated temperature with intermittent holding periods to allow thesintering aid to liquify and escape.
 10. The process of claim 9 whereinthe ramping steps are about 20° C./minute and the holding periods arebetween the ramping steps and are about one half hour.
 11. The processof claim 10 wherein there is an initial ramping step to a temperature ofless than 100° C. above the melting point of the sintering aid followedby a holding period to liquify the sintering aid, an intermediate stepto less than about 550° C. above the melting point of the sintering aid,followed by a holding period to allow vaporized sintering aid orcomponents thereof to escape, and a final ramping step to above 1500°C., followed by a holding period to fully densify the dried coatedparticles to a transparent spinel product.
 12. The process of claim 8wherein the spinel particles making the spinel dispersion have particlesize in the range of 500 nm to 100 μm; wherein the solvent includeswater and an additive selected from the group consisting of ethanol,isopropanol, and mixtures thereof; and the ratio of water to hydrocarbonto LiF sintering aid to spinel particles is about 220 ml, about 780 ml,0.2 grams, and 10 grams, respectively.
 13. The process of claim 12wherein pH of the final sintering aid solution is about 7
 14. A processfor preparing a transparent MgAl₂O₄ spinel product having maximumtransparency in excess of about 60% comprising the steps of: (a)dissolving LiF sintering aid in water to form a sintering aid solutionof about a neutral pH, (b) mixing the sintering aid solution and avolatile, low surface tension/viscosity additive selected from the groupconsisting of ethanol, isopropanol, and mixtures thereof, (c) applyingthe sintering aid solution to MgAl₂O₄ spinel particles to form a spineldispersion, (d) atomizing the spinel dispersion to form dropletscomprising at least one spinel particle coated with the final spinelsolution, (e) drying the droplets to form dried coated particlescomprising at least one spinel particle coated with a dried layer of thesintering aid, and (f) densifying the dried coated particles to form atransparent MgAl₂O₄ spinel product having uniform optical and mechanicalproperties in absence of grains of exaggerated size.
 15. The process ofclaim 14 wherein said densifying step is accomplished in a hot press byramping temperature from ambient to above about 1500° C.
 16. The processof claim 15 wherein said densifying step is accomplished by rampingsteps to an elevated temperature with intermittent holding periods toallow the sintering aid to liquify and escape.
 17. The process of claim17 wherein the ramping steps are about 20° C./minute and the holdingperiods are between the ramping steps and are about one half hour. 18.The process of claim 17 wherein there is an initial ramping step to atemperature of about 950° C. followed by a holding period to liquify thesintering aid, an intermediate ramping step to about 1200° C., followedby a holding period to allow vaporized sintering aid to escape, and afinal ramping step to above 1500° C., followed by a holding period tofully densify the dried coated particles to a transparent spinelproduct.